1. Field of the Invention
The present invention relates to a method and apparatus for ink-jet recording by using inks of a similar color each having a different density.
2 . Related Background Art
With the prevalence of information processing apparatuses such as copying machines, word processors, and computers, in addition to communication apparatuses, the use of a digital image recorder with an ink-jet recording head is spreading fast as one of these image forming (recording) apparatuses. Besides this, with the prevalence of high resolution or electrochromatic displays thereof, demand for high quality and color images is also increasing with respect to recording apparatuses. In these recording apparatuses, generally, multiple ink discharge ports and liquid paths are integrated with high density for a recording head (hereinafter referred to "multi-head") which is formed by multiple recording elements in integrated arrangement in order to speed up the recording process, and the multi-heads can provide cyan, magenta, yellow, and black colors for color recording.
However, the multi-head made by the high-density integration of ink discharge ports and liquid paths cannot entirely satisfy the requirements, which results in causing a problem for high quality image recording because of ink dots being noticeable in a highlight region of the image. Accordingly, a multi-drop recording method has been proposed as a method for acquiring high quality images by contriving an apparatus configuration: instead of increasing the density of the Integrated ink discharge ports and liquid paths, the size of the discharged ink dots is previously reduced, and then the smaller dots are duplicated several time, depending on the recording density, in the same pixels on a recording sheet. The multi-drop method slightly improves the image quality of the highlight region since it makes the dot diameter smaller than that normally available. It, however, provides limited enhancement of high quality images because of limitations on minimizing discharged ink dots due to balance against stable discharging. In addition, this method increases the number of duplication times in order to acquire the maximum density when the size of ink dots is reduced for enhancement of image quality, which lowers the recording speed significantly and causes an inconsistent relationship between the enhancement of the image quality and the recording speed.
As a method for enhancement of the image quality without increasing the integration density of the discharging port, there has been proposed a dark/light recording method which utilizes inks each having a different dyeing density and similar color; ink of a light color or light ink (ink having lower dyeing density and/or recording image density) is used for recording on highlight regions of an image to make ink dots unobtrusive and ink of a dark color or dark ink (ink having higher dyeing density and/or recording image density) is used for recording on dark regions of higher density to suppress the lowering of the recording speed.
FIG. 4 is a perspective view of one of the conventional ink-jet recording apparatuses utilizing the dark/light recording method and illustrates a main section of its configuration. In this drawing, eight ink tanks and eight multi-heads 702 are mounted on a carriage 706; the ink tanks contain dark/light inks of black, cyan, magenta, yellow, respectively, and the multi-heads are used for discharging the inks. FIG. 5 is a view of multiple nozzles arranged on the multi-heads in the z direction (viewed from the sheet) and multiple nozzles 801 are arranged on the multi-heads 702. Although the multiple nozzles 801 are arranged parallel to axis Y in the drawing, they can be arranged diagonally to some extent on the XY plane in the drawing. If so, printing is performed with different timings among respective nozzles while the heads move in a forward direction X. In FIG. 4 again, a sheet feeding roller 703 rotates the direction of the arrow in the drawing while holding a printing sheet 707 with a supporting roller 704 to feed the printing sheet 707 in the y direction according to the rotation speed. A paper feed roller 705 functions to feed the printing sheet and to hold the printing sheet 707 in the same manner as do the rollers 703 and 704. The carriage 706 is standing by at the home position (h) indicated by a dotted line in the drawing while it does not print anything or during returning of the multi-heads.
Before starting printing, when the carriage 706 at the home position in the drawing receives a printing start instruction, it moves along a carriage guide axis 708 in the x direction to print data on the sheet within the range of width D of the recording head by discharging four-color dark/light inks depending on recording signals from n multiple nozzles 801 on the multi-heads 702 with appropriate timing on the basis of the read signal of a linear encoder 709. With this recording scan, dots are formed on the sheet by jet printing of inks such as dark black ink, light black ink, dark cyan ink, light cyan ink, dark magenta ink, light magenta ink, dark yellow ink, and light yellow ink in this order. When printing data is completed at the end of the sheet, the carriage returns to the home position and then starts printing in the x direction again. During the time between the completion of the first printing and the start of the second printing, the sheet feeding roller 703 rotates in the arrow direction to feed the sheet in the y direction only by the width D. In this manner, printing data of the multi-head width D and sheet feeding are performed at every scanning by the carriage, and then printing data on a single sheet is completed after repetition of this processing.
FIG. 6 illustrates an example of an image signal processing circuit in the ink-jet recording apparatus. After color processing in a masking circuit 40 based on yellow, magenta, and cyan primary image density signals, Y1, M1, and C1, another color processing is performed by means of an under-color removal (UCR) black formation circuit 41 to convert the signals to new yellow, magenta, cyan, and black image density signals, Y36, M36, C36, and K36. Next, gamma correction is performed in a gamma (.gamma.) correction circuit 42 by using a gamma correction table in FIG. 7. Image density signals after the correction, Y37, M37, C37, and K37 are distributed to image density signals Kk38, Ck38, Mk38, and Yk38 of higher dyeing density inks such as dark black ink, dark cyan ink, dark magenta ink, and dark yellow ink, and image signals Ku38, Cu38, Mu38, and Yu38 of lower dyeing density inks such as light black ink, light cyan ink, light magenta ink, and light yellow ink in a dark/light distribution circuit 43. FIG. 8 shows one of the dark/light distribution methods. Although it is possible to calculate the signals to obtain output image density signal levels one by one on the basis of input image density signal levels, generally a dark/light distribution table based on FIG. 8 is used to speed up the processing. The dark/light distribution table is set according to the dyeing density rates so as to achieve a proportional relation between image density signal values and reflection density values after recording. After the dark/light distribution, the signals are binarized in a binarizing circuit 44 to create image signals Kk39, Ku39, Ck39, Cu39, Mk39, Mu39, Yk39, and Yu39 to be transferred to eight multi-heads 702.
As pseudo halftone processing methods with the binarizing processing which are generally known, there are a dither method, an error variance method, and an average density preservation method.
In the dither method, data of each pixel is binarized on the basis of a threshold value for each pixel determined by a dither matrix.
In the error variance method, multi-valued image data of a noted pixel is binarized (converted to the highest density level or the lowest density level) and a value of the binary level is added to a value before binarizing as described in, for example, R. FLOYD & L. STEINBERG, "AN ADAPTIVE ALGORITHM FOR SPATIAL GREY SCALE," SID 75 DIGEST, pp, 36-37.
In the average density preservation method, for example, as described in Japanese Patent Application Laid-Open No. 2-210962, threshold values are obtained on the basis of binary data around noted pixels or binary data including binarized black and white pixels which are noted, and then image data of the noted pixels is binarized on the basis of the threshold values.
Images recorded in the above methods have higher quality than the multi-drop method since light ink is used for recording data in the highlight region of an image so as to make ink dots unobtrusive and light and dark ink types are used for recording in the dark region of higher density. In the dark/light recording method, however, there are some problems caused by recording with dark/light ink of a similar color. For example, if dark/light distribution is performed as shown in FIG. 8, value 255 is always obtained as a result of addition of a dark/light output image density signal level to an input image density signal level in a region whose input image density signal level is higher than 128, therefore, the density of an inked area containing both dark and light areas of the similar color is equal to that of 100% duty. In other words, this dark/light recording consumes a large amount of ink for halftone images relative to a recording method with only dark ink, and the dark/light recording method has problems also in a halftone area which are problems in a high density inked area such as cockling of recording sheets or in an extremely high density image area in the recording method only with dark ink such as a fixing delay. The consumption of ink, however, is not a significant problem in consideration of cost performance compared with enhancement of the image quality, and the inked area density problem is not significant in comparison with the density obtained by the ink-jet recording apparatus which records data only with dark ink if the configuration is changed so that an image of 100% duty can be recorded stably.
If pseudo halftone processing is used for the dark/light recording method, the following problems may occur: If two kinds of ink each with a different density of a similar color is used for recording on the same pixel, binarizing is performed for each ink independently, which sometimes fails in acquiring the image density itself or may deteriorate the images because of the occurrence of a peculiar texture.
This is caused by the tendency of later-printed dots to sink more deeply into the recording sheet than previously-printed dots in an overlapped area when ink dots are printed over previously-printed ink dots in the ink-jet recording method. For example, if dark ink is discharged first, the density of an area having a higher rate of an overlapped area in which dark ink and light ink are discharged on the same pixels is lower than that of an area having a lower rate of an overlapped area even if an output image signal for dark ink is identical with the signal for light ink. In other words, the higher the rate of the overlapped area becomes, the more significantly the density is lowered since the light ink discharged on the dark ink dots sinks deeply and does not extend enough which does not make possible to obtain the density itself of the light ink in the area. If this phenomenon occurs at random in the recording area, partially lower density areas may appear in the recording area which must have uniform image density in itself. Further, if such areas are repeated continuously, a peculiar texture will be made, and then it leads to deteriorating the image significantly.
If binarizing is performed for dark ink and light ink independently, a problem may occur in continuation of gradation for images with gradually changing density, even if the image density is fixed in the overlapped area of dark and light ink.
FIG. 9 is a drawing illustrating the phenomena concretely. This drawing shows an example in which only black dark/light ink is used for recording to simplify an explanation, illustrating the status of printing with dark and light heads and binarizing in the simple dither method when input image density signals at a 159/255 level are entered in the dark/light distribution table.
Output image density signals are distributed into the light head side for a 191 level and the dark head side for a 64 level on the basis of the dark/light distribution table in FIG. 8, and then they are binarized independently in the simple dither method to record data in the dark/light ink recording pixel arrangement as shown in FIG. 9. The dark ink head moves for scanning specified pixels previous to scanning with the light ink head as shown in the drawing, therefore, the dark ink is discharged to form dots first, and then the light ink is discharged in order to form dots. At the positions (shown by black solid dots in FIG. 9) where the light ink is put on the dark ink dots as described in the above, the light ink is fixed as if it sinks under the surface of the dark ink dots, therefore, the dot density of the pixels is slightly increased compared with printing by using the dark ink only and it cannot obtain an increase of the density acquired from dots recorded with the light ink independently. As a result, the dots can obtain only an output image density slightly lower than the required density in the dot forming method in FIG. 9.
In the same manner, if the error variance method or other known methods are used as a binarizing method, the arrangement of the dark/light ink recording pixels is not in order as shown in FIG. 10; the overlapped dots are disordered and it causes a density difference between overlapped areas and non-overlapped areas, and further their overlapping may cause a peculiar dark and light texture.
As described above, the dark/light recording method in which upgrading images is relatively easy also has a problem regarding the overlap between the dark ink and the light ink due to causes peculiar to the image signal processing method and the ink-jet recording. Accordingly it needs improvement in order to provide high quality images.
Further in the binarizing methods such as the error variance method and the average density preservation method, errors between multi-valued image data and binary threshold values of noted pixels are distributed into pixels around the noted pixels, therefore, the distributed error is not fixed (sometimes a large error is assigned or any error is not assigned) in the binarizing of image data obtained from the dark/light distribution table and it is difficult to obtain the image density which the image data itself has.